Fix error patterns of CSSLint checker

Properly differentiate between errors and warnings.
Improve checker names

Append the name of the tool to the name of the checker unless the
checker tool is the authoritative compiler/checker for the language in
question.
[#59] Fix menu setup

`easy-menu-add-item` is not autoloaded, and apparently not considered as
part of the public interface.  Use `easy-menu-change` instead which is
nicer anyway.